A process for producing an unsubstituted coumarin dimer compound by subjecting unsubstituted coumarin to a photodimerization reaction is conventionally known. For example, Non-patent Document 1 describes an example for effecting a photodimerization reaction using benzene as a solvent, and benzophenone as a sensitizer.
Meanwhile, as a process for producing a hydroxy-substituted coumarin compound dimer compound, there has been developed a production process including: acetylating a hydroxy-substituted coumarin compound with acetic anhydride; obtaining a bis-acetoxy-substituted coumarin dimer compound through a photodimerization reaction; hydrolyzing the dimer compound with an aqueous mixture of an aqueous hydrochloric acid solution and ethanol; and extracting the hydrolyzed dimer compound with ethyl acetate and refluxing the extracted dimer compound with an acetic acid to isolate the objective dihydroxy-substituted coumarin dimer compound (Non-patent Document 2).    [Non-patent Document 1]
G. O. Schenck et. al. “Chemische Berichte” Germany, 1962, Vol. 95, pp. 1409-1412    [Non-patent Document 2]
Y. Chen et. al. “Journal of Polymer Science: Part A: Polymer Chemistry” 35, 613-624 (1997)